Conventionally, a throttle valve shaft and a throttle valve of the throttle valve assembly have been made by a metal and a throttle body of the throttle valve assembly has been made by a light metal. In recent years, a throttle body is proposed to make in such a manner that a metal member is buried to an inside wall of the throttle body made by a synthetic resin and the throttle valve is installed facing to the metal member of the inside wall of the throttle body for lightening the throttle valve assembly as disclosed in Japanese Patent Laid-Open No. 2-91431 entitled "Throttle valve body" and published on Mar. 30, 1990. When all the throttle body is made by a synthetic resin, since a thermal expansion coefficient of the synthetic resin is large, and shape and thickness of the throttle body near the throttle valve and the throttle valve shaft are complicated and comparatively thick, respectively, deformation of the body near the throttle valve and the throttle valve shaft are large corresponding to the thermal expansion and contraction and the deformation of the body is not uniform. If the worst comes to the worst, the throttle valve abuts on a wall surface of a suction air passage of the throttle body and a supporting portion of the throttle valve shaft is fixed to the throttle valve shaft.